Etching a halide glass tube, removing the etchant, and forming an optical fiber

ABSTRACT

The surface quality of halide, preferably fluoride, articles, e.g., articles used in the preparation of halide fibers is improved by cleaning the surface with an aqueous etchant and thereafter removing the etchant by washing the surface with methanol. The aqueous etchant is preferably a solution which contains hydrochloric acid and zirconium oxychloride.

RELATED APPLICATIONS

This is a division of Ser. No. 08/408,193 filed Mar. 22, 1995 (now U.S.Pat. No. 5,618,326 issued Apr. 8, 1997) which is, in turn, an FileWrapper Continuation of Ser. No. 08/157,015 filed Nov. 30, 1993 (nowabandoned) which is, in turn, a US national phase filing ofPCT/GB92/01778 filed Sep. 29, 1992.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the surface treatment of articles made fromhalide glasses and, in particular, to the surface treatment of fluorideglass articles. The surface treatment is carried out for the purpose ofimproving the surface quality.

2. Related Art

Halide glasses find particular application in the preparation of fibre,and the most usual way of preparing fibre is by stretching a suitablepreform. It has long been recognised that the surface condition of thepreform can have a substantial effect upon the performance of the fibre,and it is well established practice to treat the surfaces of halideglass articles in order to improve the performance of fibre whichresults therefrom.

The most commonly used articles in the preparation of fibres are asfollows:

Preforms

A preform consists of a region formed of the core glass surrounded by aregion formed of the cladding glass. The preform has the sameconfiguration as the fibre but it is substantially shorter andsubstantially greater in cross-sectional area. The preforms are usuallymade by casting a tube of the cladding glass and, after the cladding hassolidified but before it has substantially cooled, casting the coreglass into the bore of the tube. At this stage the whole preform isallowed to cool to room temperature. This method gives a good interfacebetween the core glass and the cladding glass, but the quality of theouter surface is often inadequate and the mechanical strength of theresulting fibre is substantially reduced. Standard preforms usually havea diameter of 10-20 mm.

Tubes

Tubes of halide glass are often utilised in the preparation of halidefibres. Tubes with 10-20 mm OD and 1-10 mm ID are particularly suitable.The tubes are used in conjunction with cast preforms as described abovewhen it is desired to reduce the core/cladding ratio of the preform(i.e. to make the core smaller in comparison with the cladding).

This technique is used when it is desired to produce fibre with smallcores. If the preform is made by casting, then it is either necessary tocast a small core into an inconveniently small bore or else it isnecessary to cast an inconveniently large cladding to produce a preformwhich has an inconveniently large diameter for fibre drawing. Thesemechanical difficulties can be avoided by casting a preform with aconvenient core size and stretching the preform so that its diameter isreduced by about 2-10 times. The diameter of the stretched preform isusually less than 10 mm and it contains too little cladding. The amountof cladding can be increased by the use of a tube of cladding glass.

Thus a cast and stretched preform can be placed in the bore of the tubeand, using techniques known to persons skilled in the art, the tube canbe shrunk until it engages with the outer surface of the preform and theouter surfaces become united, e.g. by fusion. In order for this processto operate satisfactorily it is necessary that the mating surfaces beclean and, therefore, it is necessary to provide treatments for cleaningthe surfaces. Any defects which hinder the mating of the surfaces areliable to reduce the strength and increase the attenuation of theresulting fibre.

Rods

Rods cast from halide glasses are also used in the preparation of fibreand this provides an alternative to the use of cast preforms mentionedabove. This technique comprises casting a rod of the core glass and thenshrinking a tube of the cladding glass onto the rod using the techniquementioned above. When this technique is employed, the quality of thesurface of the rod is of great importance because energy transferusually extends into the innermost regions of the cladding which meansthat the core/cladding interface forms part of the path region.Therefore defects in the interface can have an unacceptable effect uponthe attenuation of the fibre as well as having an adverse effect uponits mechanical properties. For this reason it is important that thesurface of the rod and the inner surface of the tube have high quality.It should also be realised that defects in the outer surface of the tubemay lower the tensile properties of the resulting fibre for the reasonsgiven above.

Thus, it is well established that the surface quality of articles usedin the preparation of halide glass fibres is of great importance and,therefore, surface treatments have been proposed. This treatmentsometimes includes mechanical polishing which may use abrasives. Suchtechniques are valuable when casting leaves a rough surface. Thepolishing can remove the roughness. However polishing tends to leavecontamination on the surface, for example the residues of the abrasivesmay remain on the surface. Even if mechanical polishing is not used thesurface may be contaminated and such contamination may result in thedefects enumerated above.

For this reason it is usual to clean the surface chemically, e.g. byremoving a thin surface layer of glass so as to expose a clean anduncontaminated surface. It has been found that aqueous etchantsolutions, e.g. solutions containing hydrochloric acid and zirconiumoxychloride are particularly effective for cleaning the surface of afluoride glass article. Etchants of this nature are described in U.S.Pat. No. 4,631,114.

Once the etching described above has been completed, it is necessary toremove the etchant from the surface of the glass article. This has beencarried out by washing the article in pure water and thereafter removingthe water by washing with organic solvents, which can be allowed toevaporate.

U.S. Pat. No. 4,898,777 uses an aqueous etchant based on H₃BO₃ and thisis removed by rinsing with water optionally containing HNO₃ or HCl. Avolatile solvent such as acetone or volatile alcohols (eg methanol,ethanol and isopropanol alcohol) may be used to assist drying.

We have found, most surprisingly, that the surface quality fluorideglass articles can be substantially increased by modifying the techniqueused to remove the etchant. This is particularly important where theglass article is a precursor for the preparation of optical fibre.

SUMMARY OF THE INVENTION

According to this invention the etching process is terminated by washingthe glass article with methanol. It is preferred to use a dry methanol,eg. having a water content below 0.01% w/v, (i.e. less than 0.01 gms ofwater per 100 ml of solution). For example the article is transferred toa bath of methanol where it remains for long enough to remove theetchant. The methanol will evaporate, e.g. after the article is removedfrom the bath, and this evaporation causes no deterioration in thesurface quality of the article. The aqueous etchant is preferably aconventional etchant, eg as mentioned above, namely an aqueous solutionof hydrochloric acid and zirconium oxychloride.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a configuration of preformsaccording to the invention; and

FIG. 2 shows steps involved in a method of the invention.

The technique of the invention can be applied to the complete range ofhalide glasses wherein at least 90%, and preferably all, of the halideis fluoride. The metals which are used to make the fibre may be selectedfrom Zr, Hf, Ba, La, Al, Li, Na, K, Pb, Cs, Bi, Be, Y and the rareearths. (The rare earths are lasing dopants.) Preferred glasses containhalides of Zr and/or Hf together with halides of Ba and, optionally, atleast one other halide from the above list.

Some specific embodiments of the invention will now be described by wayof example. Four different fluoride glass compositions were used inthese examples. The four compositions are identified in Table I.

TABLE I GLASS A B C D ZrF₄ 53 51.5 40 51.8 HfF₄ — — 13.3 — BaF₂ 20 19.517.9 19.5 LaF₃ 4 5.3 4 4.6 AlF₃ 3 3.2 2.9 3.2 NaF 20 18 21.9 18 PbF₂ —2.5 — — RI 1.4985 1.5056 1.4935 1.5075

The numbers quoted (except refractive indices) in Table I represent molepercent of the relevant component.

The compositions defined in Table I can also be used as hosts fordopants, e.g. metal fluorides. Dopants are used to confer activeproperties on the composition. Active properties include lasing for usein fibre amplifiers and varying the refractive index for use inswitching devices. The rare earths, especially Er, Nd and Pr, Ho and Ybare particularly suitable as dopants.

The same aqueous etchant was used in all the examples and thecomparison. This etchant consisted of 0.4 molar ZrOCl₂ 8H₂O and 1 molarHCl in water.

EXAMPLE 1

Two similar preforms were prepared using a conventional castingtechnique in which a tube of Glass A was centrifugally cast and Glass Bwas poured into the bore of the tube. Glasses A and B are defined inTable I. Each preform was 100 mm long. Diameter of the core was 5 mm andthe cladding was 2.5 mm thick, so that the diameter of the preform was10 mm.

The surface of each preform was cleaned by etching with the aqueousetchant defined above. This etching comprised immersion for 25 minutesat ambient temperature (about 20° C.). After immersion each of thepreforms was washed to terminate the etching. One preform (selected atrandom) was subjected to a methanol wash in accordance with thisinvention whereas the other preform was washed in (pure) water followedby washing in isopropanol.

In accordance with the invention, the washing comprised 15 minutesimmersion in (1 liter) pure methanol. During this immersion the preformwas spun while the methanol was subjected to ultrasonic agitation.Afterwards the methanol was allowed to evaporate by agitation in air.

The conventional wash comprised immersion in pure water with spinningbut without ultrasonic agitation. After washing in water the preform waswashed in a bath of isopropanol while the isopropanol was subjected toultrasonic agitation. This replaced the residual water by residualisopropanol which was allowed to evaporate by agitation in air.

After etching and washing as described above, each of the preforms waspulled into fibre and 20 samples were cut from each of the fibres forthe measurement of breaking strain. The percentage breaking strain ofeach sample is given in Table II in which the breaking strains arearranged in order of magnitude.

TABLE II METHANOL WATER/ISOPROPANOL 2.3 1.8 2.4 1.8 2.5 1.9 2.5 1.9 2.62.0 2.7 2.0 2.8 2.0 2.9 2.0 3.1 2.1 3.1 2.2 3.1 2.2 3.2 2.3 3.2 2.3 3.22.3 3.3 2.4 3.3 2.5 3.3 2.6 3.4 2.7 3.7 2.8 4.2 2.9

The median value of the breaking strain for samples washed in methanolwas 3.1% which compares with 2.2% for samples washed in water inaccordance with the prior art. This is a significant and valuableimprovement in the breaking strain. It will also be noted that the bestvalues obtained with the conventional water wash are slightly less thanthe median values obtain in accordance with the invention.

It was also observed that the fibre washed in accordance with theinvention had a more uniform diameter than fibre washed with water inaccordance with the prior art.

Example 1 gives etch and wash conditions suitable for preforms or rodshaving an external diameter in the range 10-20 mm. For preforms and rodshaving a diameter less than 10 mm, it is recommended that the etchingtime be reduced, e.g. to about 4 or 5 minutes followed by 10 minutes forrinsing in methanol. In the case of tubes, e.g. tubes with outsidediameters in the range 10-20 mm, and internal diameters in the range of1-10 mm, the following two stage procedure is recommended. The firststage comprises treating the tube in the same way as a standard preformdescribed above. The second stage comprises passing aqueous etchantthrough the bore of the tube using turbulent flow. After 5 mins,methanol is added to the etchant in increasing concentrations until, atthe end of the procedure, the bore is washed with pure methanol. Thewhole procedure takes about 20 minutes.

In addition to the improved breaking strains as mentioned above, it wasalso observed that the attenuation of the fibre was decreased.

EXAMPLE 2

This example describes the preparation of a preform with a small corefor conversion to monomode fibre. It is difficult to make small cores bythe method described in Example 1 because it is difficult to cast a tubewith a small bore.

A tube of Glass C (cladding) was centrifugally cast and a compositionbeing Glass D doped with ErF₃ was cast into the bore of the tube wherebyDoped Glass D formed the core and Glass C formed the cladding of a firstpreform. The diameter of the core was 5 mm and the cladding was 2.5 mmthick so that the external diameter was 10 mm. The first preform, whichwas 100 mm long, was stretched to reduce the cross-sectional area ofboth the core and the cladding, although the ratio

(area of core):(area of cladding)

was not altered. The diameter of the core was reduced by this stretchingto 0.5 mm, which implied that the first preform was stretched to alength of 10 M and its external diameter was reduced to 1 mm. In orderto restore the original diameter of 10 mm, a segment, 100 mm long, ofthe stretched first preform was jacketed with a tube of Glass C. Thejacketing was carried out using conventional glass handling techniquesin which the stretched, first preform was placed inside the bore of thetube and the tube was shrunk to make contact with the preform. Theprocess was carried out by heating to a temperature sufficient to softenthe glass so that the mating surfaces became unified by fusion. It is,however, important that the mating surfaces have good quality, becauseimperfections and impurities substantially degrade the performance ofthe product.

Before jacketing, the surfaces were cleaned and washed in accordancewith the invention. The tube and the first preform were both etchedusing the solution specified above. As described in Example 1, the tubewas placed first in an etchant bath and then in a washed in a bath ofmethanol. Thus the outer and inner surfaces of the tube were both etchedand then washed simultaneously. The inner surface of the tube was thenre-etched by using turbulent flow to pass etchant and washing solutionsthrough the bore. Initially, the solution was the pure etchant butmethanol was added to the etchant solution which was gradually diluteduntil pure methanol was flowing through the bore of the tube. Thisprocess took 15 minutes, after which the residual methanol was allowedto evaporate.

The stretched preform was spun for 1½ minutes in the etchant, and thenrinsed in 1 liter of methanol for 5 minutes. The etched tube was thenshrunk onto the etched preform as described above.

It was found that washing with methanol in accordance with the inventiongave a good interface in the preform which could be stretched to goodquality fibre. In comparative experiments, using water/isopropanol forwashing, it was found that the surface was degraded and it was verydifficult to achieve a good mating. In these comparative experiments, itwas very difficult to draw the preforms into fibre and the quality ofthe resulting fibre was not assessed.

The configuration of the preforms, and the fibres resulting from them,is illustrated in the drawing.

As shown in the drawing, the preform produced by Example 2 comprises acore 10 which is surrounded by an inner cladding 11, which, in its turn,is surrounded by an outer cladding 12 which had the same composition asthe inner cladding 11. The interface 13 between the core and the innercladding was produced by casting a molten glass into the bore of a hottube and the washing of the invention is not relevant to this interface.The interface 14 between the inner and outer cores was produced byshrinking the tube onto the first preform and the quality of thisinterface was improved by washing in accordance with the invention. Theouter surface 15 was also washed in accordance with the invention, and,as shown in Table I, the quality of this surface affects the mechanicalstretch of the fibre.

The method described in Example 2 is also applicable to rod-in-tubeprocesses for making fibre. In this case there is no interfacecorresponding to 14 but the interface 13 is produced by uniting surfacesand washing in accordance with the invention is applicable to thisinterface.

In order to appreciate the improvement produced by the invention, itshould be remembered that, while most of the energy transfer occurs inthe core, a small proportion of transfer occurs in the inner layers ofcladding. It is, therefore, to be expected that improving the quality ofthe core/cladding interface 13 will have a direct and beneficial effecton the attenuation of the fibre. The improved surfaces achieved bywashing in accordance with the invention therefore have a direct effectupon attenuation where the core/cladding interface is produced by a rodin tube process.

In the case, illustrated in Example 2, where a tube is shrunk onto asmall preform the new interface 14 is located in the in the interior ofthe cladding where there is little or no energy transfer. It is apparentthat such a surface could affect the mechanical strength of theresulting fibre, but it may be surprising that there is an effect on theattenuation because there is no energy transfer at this interface.Nevertheless, in the softened state used for drawing, mechanicalimperfections are transferred through the inner layers of cladding, andnoticeable effects are produced in the region where energy transfertakes place. Therefore, even in this case, the surface quality has aneffect upon attenuation and washing in accordance with the inventionimproves the attenuation.

By way of comparison, the technique of the invention was attempted withorganic solvents other than methanol. Ethanol and isopropanol were usedin these comparative tests. Each test comprised cleaning the surface ofan article made from one of the glasses specified in table 1 abovewherein said cleaning was carried out using an aqueous etchant whichconsisted of 0.4 molar zirconium oxychloride (ZrOCl₂. 8 H₂O) and 1 molarHCl. (This is the same etchant that was used in example 1 and its use isconventional.) After etching the etchant was removed, in some tests,with isopropanol and in the other tests with ethanol. Not one of thesetest give a satisfactory result. These tests can be regarded either as amodification of the invention in which the use of methanol is replacedeither by ethanol or isopropanol or they can be regarded as amodification and a conventional technique in which the water wash iseliminated.

The results given by ethanol and isopropanol were unsatisfactory becausethere was a visible deposit on the surface of the glass article. Asexplained above, etching is carried out in order to clean the surface.If the cleaning technique leaves a dirty surface it is apparent that thecleaning technique is unsatisfactory

Thus, the invention pertains to a method of making a fibre waveguidefrom halide glasses. The method comprises preparing a fibre preform andthereafter drawing the preform into the fibre. The preparation of thepreform comprises:

(I) casting a tube of halide cladding glass,

(II) casting a halide core glass in the bore of the tube to produce apreform having an outer surface, and

(III) cleaning the outer surface of the preform.

The cleaning (III) comprises:

(a) immersing the preform in an aqueous etchant whereby the outersurface is etched,

(b) washing the aqueous etchant from the outer surface by means ofmethanol whereby the etching is terminated, and

(c) allowing the residual methanol to evaporate.

Preferably all of the halide glass is a fluoride glass and the methanolcontains less than 0.0% w/v of water.

The invention also concerns a method of making a fibre waveguide fromhalide glasses, which method comprises preparing a fibre preform andthereafter drawing the preform into the fibre. Preparation of thepreform comprises:

(i) preparing either

(x) a rod of a halide core glass, the rod having an outer surface, or

(y) a rod comprising an inner portion of a halide core glass surroundedby an outer portion of a halide cladding glass, the outer portion havingan outer surface constituting the outer surface of the rod;

(ii) preparing a tube of a halide cladding glass, the tube having aninternal and an external surface,

(iii) cleaning each of the external, internal and outer surfaces, andthereafter,

(iv) shrinking the tube onto the rod whereby the internal surface mateswith the outer surface of the rod.

The cleaning comprises, in each case,

(a) immersing the tube and the rod in an aqueous etchant whereby all ofthe surfaces are etched,

(b) washing all of the etchant off all of the surfaces by methanolwhereby the etching is terminated, and

(c) allowing the residual methanol to evaporate.

Preferably all of the halide glass is a fluoride glass, and the methanolcontains less than 0.01% w/v of water.

The invention further includes a method of improving the surface of ahalide glass article, which method comprises immersing the article in anaqueous etchant and thereafter washing the article with methanol toremove the etchant, and allowing the residual methanol to evaporate fromthe surface of the article. The halide of the glass is preferably atleast 90 mole % fluoride, and the remainder of the halide is chloride.The halide of the glass can be 100% fluoride. The glass preferablycontains at least one halide of Zr, Hf, Ba, La, Al, Li, Na, K, Pb, Cs,Bi, Be and Y and a rare earth or other active dopant. A weekly acidicaqueous etchant can be used, such as an aqueous etchant containinghydrochloric acid and zirconium oxychloride. The halide glass article isselected from tubes of halide glass, rods of halide glass and fibrepreforms comprising a core of a first halide glass surrounded by and incontact with a cladding of a different halide glass. The methanolcontains less than 0.01% w/v of water.

What is claimed is:
 1. A method of improving a surface of a bore of atube of a halide glass, the method comprising: passing an aqueousetchant solution through the bore of the tube whereby the surface of thebore is etched; and terminating said etching by initially addingmethanol to the aqueous etchant solution and passing themethanol-containing aqueous solution through the bore of the tube andthereafter increasing the concentration of the methanol in the aqueoussolution thereby decreasing the concentration of the etchant in theaqueous solution until, over time, pure methanol flows through the boreof the tube, whereby etching is terminated and etchant is removed fromthe surface of the bore; and evaporating the methanol from the surfaceof the bore.
 2. A method of making fiber waveguide which methodcomprises preparing a fiber preform and thereafter drawing said preforminto a fiber, wherein said preparation of said preform comprises: (i)preparing a rod of a halide core glass, said rod having an outersurface; (ii) preparing a tube of a halide cladding glass, said tubehaving a bore defined by an internal surface of the tube, (iii) cleaningthe bore of the tube, and thereafter, (iv) shrinking said tube onto saidrod whereby said internal surface mates with the outer surface of saidrod, wherein said cleaning comprises: passing an aqueous etchantsolution through the bore of the tube whereby the surface of the bore isetched; and terminating said etching by removing etchant from the bore,said removal comprising initially adding methanol to the aqueous etchantsolution and passing the methanol-containing aqueous solution throughthe bore of the tube and thereafter increasing the concentration of themethanol in the aqueous solution thereby decreasing the concentration ofthe etchant in the aqueous solution until, over time, pure methanolflows through the bore of the tube, whereby etching is terminated andetchant is removed from the surface of the bore; and evaporating themethanol from the surface of the bore.